1. Field of the Invention
This invention relates to a method and apparatus for receiving a radio signal, in which blind-switching-diversity may be implemented in a receiver of digitally-encoded data, such as time division multiplexed/time division multiple access (TDM/TDMA) data. The invention finds particular application in the field of fixed wireless access (FWA).
2. Description of the Prior Art
In wireless TDM/TDMA communications networks, message data are transmitted in channels comprising predetermined time slots within a series of fixed-length frames. Each frame or slot contains a synchronization (or sync) word comprising a sequence of predetermined sync symbols. Sync words are used by a receiver for timing and carrier (phase and frequency) synchronization. This includes equalizer training, which is the process of adapting the filter coefficients (or tap weights) of an equalizer so as to adjust its phase and/or frequency response to compensate as far as possible for multipath interference or channel distortion in the wireless transmission.
Other receiver functions can also be trained using sync words, such as carrier phase recovery, time slot recovery and/or automatic gain control.
In a conventional TDM/TDMA receiver, each slot carrying a sync word and data is demodulated starting with the sync word, so that the equalizer and any other relevant functions may be trained before recovery of the received data. This is to reduce the occurrence of errors in data recovery.
The time taken to train an adaptive equalizer, or other receiver function, depends on the lengths of the sync words and on the complexity and effectiveness of the method used to adapt the filter coefficients when a sync word is received. However, if the sync word is too short it may not be possible to train the equalizer fully using only one sync word. It is then necessary to train the equalizer iteratively, using the sync words in a series of slots or frames. Of course training may then take a significant time and data received in early frames may not be recovered accurately. In fact, until the equalizer is trained it is likely to be impossible to recover data at all. This condition of the receiver may be termed a closed-eye pattern.
It is possible to ensure rapid training, often even within one slot, by using longer sync words. This provides rapid training but disadvantageously reduces the amount of message data which can be carried in each slot.
The balance between providing sync words of adequate length and maximizing the data capacity of each slot depends significantly on the type of communications network involved. For example in a mobile communications network, such as a cellular mobile telephone network, multipath interference and other channel distortion may be severe and, significantly, may change very fast. This means that a TDM/TDMA transmission for mobile communications must contain sufficiently long, sufficiently frequent, sync words to allow equalizer training during reception of the transmission in order to compensate for rapid channel-distortion variation. For example, if channel distortion changes significantly between the slots of a channel in two consecutive frames, then unless the equalizer can retrain sufficiently using the sync word at the start of the slot in the second frame, it may not be possible to recover the data in the second frame. However, the presence of a long sync word in every slot is a system overhead which can significantly reduce useful message data capacity.
One way to reduce sync word overhead in some systems is to use a longer sync word in a pilot frame or slot at the beginning of a new transmission, to allow rapid equalizer training when the transmission is set up, and shorter sync words in subsequent frames. Nevertheless, the sync words in subsequent frames in mobile communications systems must still be long enough to allow rapid training during reception of the transmission.
In fixed wireless access (FWA) communications, radio signals are transmitted between a fixed base station and fixed subscriber units. For example, a FWA link may form part of a telephone system replacing the local link between an exchange and each subscriber's home or business. Therefore, in a FWA link, channel distortion varies much more slowly than in mobile communications.
Channel distortion may still vary during a transmission, so equalizer training during the transmission is still required, but it is much less likely that the channel distortion may change enough during a single frame to prevent accurate reception of data in the following frame.
Therefore, in a FWA system shorter sync words than those needed in a mobile system can provide sufficient retraining between frames to maintain good reception. Using shorter sync words also advantageously reduces the system overhead and can significantly increase the amount of useful message data which can be carried.
Certain problems arise, however, from the use of short sync words. For example, when a communication is newly set up, if short sync words are used it may take several frames for the receiver equalizer to train fully to the new transmission and so data may be lost at the beginning of the transmission. Starting each new transmission with a pilot frame or slot carrying a longer sync word can alleviate this problem.
A problem also arises in implementing diversity reception, in which performance is enhanced by using two or more receiving antennas. A low complexity form of diversity is blind switching, in which a switch to a new antenna is made if the signal quality derived from, or the signal level received by, a presently-used antenna becomes unacceptable, but in which the signal quality or signal level from the new antenna is not known before switching. The signal quality or signal level from the new antenna is only known after switching.
Blind switching diversity is an effective means of providing the advantages of diversity at low cost, as only a single set of receiver circuitry is needed. There is no requirement to monitor signals received by the new antenna before switching. However, when a switch is made to a new antenna with no prior knowledge of the signal quality receivable by that antenna, there is a risk that it will be worse than the signal quality from the previously-connected, or old, antenna. If this happens, then a decision to switch back to the old antenna or, if available, to another new antenna, must be made as quickly as possible.
When a switch to a new antenna is made, the channel distortion is usually different from that of the old antenna, so the equalizer (and any other relevant components of the receiver) must retrain to compensate for the new channel distortion.
This retraining causes no difficulty in a mobile communications system which uses sufficiently long sync words to allow rapid retraining at any time during a transmission. If a blind switch is made to a new antenna in a mobile system, the receiver can typically retrain within one slot or frame. It can therefore recover data immediately from the new signal and assess its quality without delay, allowing a rapid decision to be made to switch back to the old antenna if the signal quality from the new antenna is insufficient. Even if the signal quality from the new antenna is so poor that data from the signal cannot be decoded, no more than a single frame (or slot) of data need be lost, which usually causes only an insignificant interruption in communication. Consequently, blind switching diversity in mobile communications receivers is well known.
By contrast, if blind switching diversity is considered for a FWA system using short sync words to optimise data capacity, retraining to the signal received by a new antenna after a blind switch during a transmission may take several frames. A significant delay would then be expected before data could be recovered, the quality of the data evaluated and a decision made whether or not to switch again to the old antenna or to another new antenna. Such a delay could disadvantageously cause a significant interruption in communications.
Nevertheless, diversity reception in FWA systems is desirable because, although channel fading occurs relatively slowly, if a channel does fade sufficiently that message data is lost then it may take a long time for the channel distortion to change so that communication can be resumed.